8 : 3/ Muscles 



145 



probably acetylcholine, which is probably also important in the trans- 

 mission of impulses across synapses between nerves. (Acetylcholine and its 

 action are described more completely in Chapter 4.) The released acetyl- 

 choline diffuses across the myoneural junction (which is of the order of a 

 few tenths of a micron) and stimulates the formation of a spike potential 

 in the muscle fiber. The acetylcholine is rapidly destroyed by a protein 

 catalyst, cholinesterase, present in the muscle end plate. Under certain 

 conditions, the myoneural junction acts as a "computer," putting out a 

 number of muscle spike potentials different from the number of incoming 

 nerve spike potentials. 



The muscle fiber membrane is polarized, just as is the axon membrane 

 discussed in Chapter 4. An action spike potential, similar to that in 



-90mv 



Figure 6. Spike potential of striated muscle. V is the poten- 

 tial difference inside minus that outside the sarcolemma. 

 The arrow indicates application of stimulus. In cardiac 

 muscle, the peak of the crest of the action potential lasts much 

 longer. 



nerves, is the first result of stimulation of a muscle fiber, whether the 

 stimulus be the physiological one from the nervous system or an arti- 

 ficial one, that is, electrical, mechanical, or heat. A typical muscle 

 spike potential is shown in Figure 6. The action potential differs from 

 that in nerves only in the duration of the peak, which lasts much longer 

 in muscle than in nerve. 



Originally, the muscle potentials were recorded by means of so-called 

 "bipolar" or "differentiating" electrodes which measured the potential 

 difference between two neighboring spots on the muscle. These gave 

 no possibility of measuring a resting or d-c potential, nor any certainty of 

 the size of the cellular potentials. These electrodes have been replaced 

 by microelectrodes made by drawing out a capillary glass tube to a 

 diameter of less than 1 /x. The tiny capillaries may be inserted through 

 the wall of a single muscle fiber without damaging the fiber. With such 

 probes, it is possible to measure both the resting potential and the action 



